It is well known that spontaneous emission sets a quantum limit to the laser linewidth. There are presently phase or frequency measurements where such a quantum limit has been reached. Therefore, a range of applications including metrology, high resolution spectroscopy, rotation or spatial anisotropy sensing by active ring lasers would benefit from any improvement of the influence of spontaneous emission noise on the measurement. The direct quantum solution to this problem is to minimize the phase noise at the expense of the noise in the conjugate quantities. Examples of implementation of the quantum solution are "correlated emission lasers" and "squeezed state" sources.
The instant invention provides an approach wherein the spontaneous emission is interferometrically decoupled from the quantity measured from the laser. The background of this approach is discussed in detail in an article entitled "Ring-laser Configuration with Spontaneous Noise Reduced by Destructive Interference of the Laser Outputs," M. Lai and J.-C. Diels, The Physical Review A, pp. 536-542, Vol. 42, No. 1 (1990) which is incorporated herein by reference. This technique takes advantage of the fact that spontaneous and stimulated emissions may have different symmetries in the opposite propagation directions of a ring laser cavity. The concept of the instant invention applies to gain media consisting of thin layers (where thickness D&lt;&lt;.lambda.), for which the spontaneous emission can be minimized by destructive interferences. Even though the spontaneous emission from an atom or molecule is random, the oppositely directed emissions from an electrical dipole are exactly identical. That is, at the same distance from the source, the electromagnetic fields emitted in opposite directions have, at any instant, equal amplitude and phase. Therefore, they can be made to recombine and interfere via a pair of mirrors and a beam splitter. For an ideal destructive interference from a sheet of emitters of thickness D (where thickness D&lt;&lt;.lambda.), the spontaneous emission minimum intensity (measured at one output of the beam splitter) is the fraction {1-[sinkD/kD]}/2 of the total florescence intensity at that point. The destructive interference condition still holds if the single emitting layer is replaced by a periodic structure of layers spaced by .lambda./2, as long as the total thickness is much less than the coherence length of the florescence radiation. If .DELTA..lambda. is the radiation linewidth (half width at half maximum), a series of (2N+1) layers with thickness D will result in a minimum florescence intensity (after destructive interference) of: ##EQU1##
When the excited atoms are spatially confined in a ring laser cavity and located at half way of the perimeter from the output mirror, the two oppositely directed spontaneous emissions seen from the output mirror should be identical. On the other hand, the two counterpropagating waves of the ring laser may be different in amplitude or phase, due to the laser dynamics, such as mode competition. In particular, they may have different frequencies due to the gyro effect or due to magnetic field sensing. For example, for magnetic field sensing an embodiment of the invention may employ the well-known Verdet effect in one element of the cavity, or, alternatively employ well-known Zeeman splitting of some absorbing-amplifying elements in the cavity. The two outputs can then be combined, via a pair of mirrors and a beam splitter, in such a way that, at one of the beam splitter outputs, an optimum destructive interference is obtained for the spontaneous emission (FIG. 1). Within the approximation of a small number of thin (D&lt;&lt;.lambda.) gain layers, the laser beam from that output is free from the spontaneous emission.